The goal of the laboratory is to understand the cell biology of epidermal Langerhans cells (LC). We have completed our studies of expression of the costimulatory molecule B7 by epidermal cell (EC) subpopulations. Using primers specific for murine B7 and quantitative RT-PCR, we determined that B7 mRNA was 100-1000 fold more abundant in 72 h cultured BALB/c EC (LC and keratinocytes) than in fresh EC. Cell depletion studies carried out with monoclonal antibodies and magnetic beads demonstrated that the B7 mRNA present in cultured EC was contained in LC. Using identical conditions, essentially no B7 mRNA was identified in freshly prepared EC (LC and keratinocytes), normal or transformed keratinocytes, or fibroblasts. Two color flow cytometry, carried out with the B7-binding fusion protein CTLA4Ig, confirmed that cultured LC express abundant surface B7 while fresh LC and keratinocytes do not. These data suggest that among EC, LC are uniquely able to synthesize and express B7. In addition, levels of B7 expressed by LC are proportional to mRNA levels. More recently, we discovered that LC express E-cadherin (E-cad), one of a supergene family of calcium-dependent homophilic adhesion molecules. We also demonstrated that LC contain E-cad mRNA and that E-cad mediates adhesion of LC to keratinocytes. E-cad is not expressed by splenic or gut-associated lymph node (LN) dendritic cells (DC), but may be expressed at low levels by subpopulations of thymic DC and DC prepared from skin-associated LN. In addition, E-cad is expressed by DC propagated from blood and bone marrow. These results suggest that E-cad is a DC differentiation antigen expressed by LC, cells that may be derived from LC (skin-associated LN DC), cells that may give rise to LC (blood and bone marrow DC), and cells that may be closely related to LC (thymic DC). We have begun to use blood and bone marrow-derived DC as a model to study the biochemistry and cell biology of leukocyte E-cad. We have demonstrated that DC contain mRNA identical to that encoding E-cad expressed by nonleukocytes. E-cad immunoprecipitated from DC also comigrates with that derived from fibroblasts transfected with E-cad cDNA. We are trying to identify factors that regulate DC E-cad expression and activity and will use E-cad to study LC and DC ontogeny. We have also detected E-cad on the surfaces of resident murine epidermal gamma-delta T cells and fetal thymocytes, suggesting that E-cad may play a more general role in leukocyte-epithelial interactions. Additional experiments will be required to determine whether or not E-cad is involved in adhesion of gamma-delta T cells to keratinocytes, and to determine if E-cad plays a role in thymocyte development.